Handling colons in ipv6

[devikasondhi]

Invalid address 1111: is flagged as valid by the ipv6 validator

[devikasondhi]

Cases with leading colons are also not handled for cases as below:
":8888"
":1.2.3.4"

These are invalid ipv6 addresses but the validator on ipv6 returns True

[kvesteri]

PRs welcome

[SimonIT]

"18:05" returns also True